Silicon based power devices have long dominated power electronics and power system applications. On the other hand, SiC is a wider band-gap (Eg) material with Eg=3.3 eV as compared to silicon (Eg=1.1 eV) and hence, SiC has a higher blocking voltage than Si. SiC has a higher breakdown electric field (3×106 V/cm to 5×106 V/cm) compared to Si (breakdown electric field for Si is 0.3×106 V/cm) and is a better thermal conductor (3.7 (W/cm-K) for SiC versus 1.6 (W/cm-K) for Si). SiC has been a material of choice for power MOSFETs. However, “[e]ven with the successful introduction of SiC power MOSFETs into the commercial market place, several key reliability issues have not been fully resolved.” [Source: Key Reliability Issues for SiC Power MOSFETs, A. Lelis, D. Habersat, R. Green, and E. Mooro of the U.S. Army Research Laboratory, published in ECS Transactions, 58 (4) 87-93 (2013), DOI: 10.1149/05804.0087ecst]
“[W]hile SiC power MOSFETs share many similarities to silicon MOSFETs, many challenging differences remain. In particular, the wide-bandgap nature of 4H-SiC (EG=3.26 eV) is both a blessing and a curse, bringing a low intrinsic carrier concentration and a high critical electric field, while presenting challenges with inversion-layer mobility and reliability in passivating dielectric layers.” [Source: Challenges in SiC Power MOSFET Design by Kevin Matocha of the GE Global Research Center—Semiconductor Technology Laboratory, Niskayuna, N.Y. USA, published in ISDRS 2007, Dec. 12-14, 2007, College Park, Md., USA]
SiC exists in a kind of polymorphic crystalline building known as a polytype, e.g. 3C—SiC, 4H-SiC, 6H-SiC. FIG. 1 is the prior art SiC DMOSFET structure reported by B. J. Baliga in Advanced High-Voltage Power Device Concepts (Springer Press, 2011).
In a typical SiC planar MOSFET structure as that shown in FIG. 1 the metal oxide semiconductor (MOS) channel is formed on the horizontal or 0001 crystal plane of SiC. The channel mobility or field effect mobility on the 0001 crystal plane of SiC tends to be lower as compared to the vertical side wall or what is called as the 11-20 or 10-10 crystal plane. The channel mobility or field effect mobility on the 0001 crystal plane of SiC is in the range of 15 to 25 cm2/V-sec as compared silicon MOSFETs where it can be more than 350 cm2/V-sec. When one makes a planar SiC DMOSFET, the ON resistance tends to be large and the MOSFET is limited by the field effect channel mobility. Therefore, there is a long-felt need for improved power devices that addresses the low channel mobility and high ON resistance in SiC MOSFET.